Recently, from the viewpoint of effective utilization of energy aimed at global environmental conservation and resource saving, a power smoothing system for wind power generation or a nighttime power storage system, a home-use distributed electrical storage system based on photovoltaic power generation technology, an electrical storage system for electric vehicles, etc., have attracted attention.
In these electrical storage systems, the first requirement is that the energy density of a battery to be used is high. As a strong candidate of a battery having high energy density capable of satisfying the requirements, lithium ion batteries has been actively developed.
The second requirement is high output characteristics. For example, in a combination of a highly efficient engine and an electrical storage system (for example, a hybrid electric vehicle), or in a combination of a fuel cell and an electrical storage system (for example, a fuel-cell electric vehicle), high output discharge characteristics are required from the electrical storage system, in order to achieve acceleration.
At the present time, as a high-output storage element, electric double layer capacitors using an activated carbon as electrodes, have been developed, and exhibit an output characteristics of about 0.5 to 1 KW/L. These electric double layer capacitors have high durability (cycle characteristics and high temperature storage characteristics), and have been considered to be the optimum device in fields where the above-described high output is required, however, the energy density thereof is only about 1 to 5 Wh/L, and further improvement of the energy density is required.
On the other hand, a nickel-hydrogen battery that is currently adopted for use in hybrid electric vehicles has the same high output as that of the electric double layer capacitors, and has an energy density of about 160 Wh/L. However, research is being actively carried out to further enhance energy density and output thereof, as well as to further improve stability at high temperatures and enhance durability.
In addition, as with lithium ion batteries, research continues toward realizing higher output. For example, a lithium ion battery has been developed that is capable of providing a high output of over 3 kW/L, at a depth of discharge (a value indicating a state of discharge of the storage element in terms of percentage) of 50%. However, a lithium ion battery has been actually designed to suppress high energy density equal to or less than 100 Wh/L, even though a lithium ion battery is identically characterized by a high density. In addition, durability thereof (cycle characteristics and high temperature storage characteristics) is inferior to that of the electric double layer capacitors. Therefore, in order to have practical durability, the lithium ion battery is usable only in a depth of discharge that is a narrower range than 0 to 100%. Therefore, usable capacitance in practice is reduced, and further research is being carried out to enhance the durability.
Although practical application of the storage element having all of high energy density, high output density, and durability, as described above, has been strongly required, the above-described existing storage elements have advantage and disadvantage. Accordingly, a new storage element satisfying these technological requirements has been required, and as a strong candidate thereof, the storage element called a lithium ion capacitor has attracted an attention and has been actively developed.
Energy of a capacitor is expressed by ½·C·V2 (wherein, C is static capacitance, and V is voltage). A lithium ion capacitor is one type of a storage element (nonaqueous lithium-type storage elements) that uses a nonaqueous electrolytic solution containing a lithium salt, and carries out charge/discharge by a non-faradaic reaction based on adsorption/desorption of a negative ion similarly as in the electric double layer capacitor, in a positive electrode at about 3 V or higher, and by a faradaic reaction based on intercalation/deintercalation of lithium ions similarly as in the lithium ion battery, in a negative electrode.
As described above, in the electric double layer capacitors which carries out charge/discharge by the non-faradaic reaction in both the positive electrode and the negative electrode, output characteristics are superior (it means that charging and discharging of high current are possible in a short period of time), but energy density is low. On the other hand, in a secondary battery which carries out charge/discharge by the faradaic reaction in both the positive electrode and the negative electrode, energy density is superior but output characteristics are inferior. The lithium ion capacitor is the storage element aimed at compatibility of both superior input/output characteristics and high energy density, by carrying out charge/discharge based on the non-faradaic reaction in the positive electrode, and based on the faradaic reaction in the negative electrode.
As examples of the lithium ion capacitor, there has been proposed a storage element using an activated carbon as a positive electrode active material and a carbonaceous material as a negative electrode active material, wherein the carbonaceous material is a carbon material capable of accommodating/releasing lithium in an ionized state, to which lithium is accommodated in advance by a chemical method or an electrochemical method, and includes natural graphite, artificial graphite, graphitized mesophase carbon microsphere, graphitized mesophase carbon fiber, graphite whisker, graphitized carbon fiber, a pyrolysate of a furfuryl alcohol resin or a novolac resin, or a pyrolysate of a polycyclic hydrocarbon condensed polymeric compound, such as pitch or cokes, etc. (see PATENT LITERATURE 1).
In addition, as shown below, there has been proposed an electrode and/or a storage element using an activated carbon as a positive electrode active material, and a carbonaceous material as a negative electrode active material, wherein the carbonaceous material is a composite porous material, in which a carbonaceous material is deposited on a surface of an activated carbon, and to which lithium is accommodated in advance (hereafter it may also be referred to as “pre-doping” to distinguish from the accommodation “dope” and release “undope” of lithium ions generated at a negative electrode in charge/discharge to the negative electrode) (see PATENT LITERATURE 2 to 6). The lithium ion capacitor using the composite porous material for a negative electrode is characterized by having lower internal resistance, because it has larger surface area as compared with a lithium ion capacitor using other materials, such as graphite, for the negative electrode.
PATENT LITERATURE 2 describes an electrode having a discharge capacitance (referred to as B) of 605 mAh/g, and an initial efficiency (determined by B/A) of 56%, by electrochemically pre-doping lithium (the pre-doped amount is referred to as A) to a negative electrode active material which has a weight ratio of a carbonaceous material to an activated carbon (hereafter it may also be referred to as “the weight ratio”) of 50%.
PATENT LITERATURE 3 describes an electrode having, by pre-doping lithium electrochemically to a negative electrode active material where the weight ratio is 50% or 29%, a discharging capacitance (B) of 605 mAh/g, and an initial efficiency (B/A) of 56%; and an electrode having a discharging capacitance (B) of 560 mAh/g, and an initial efficiency (B/A) of 51%. PATENT LITERATURE 3 also describes a lithium ion capacitor, having a negative electrode to which lithium is pre-doped electrochemically, in an amount of 1000 mAh/g or 500 mAh/g, to a negative electrode active material where the weight ratio is 50%.
PATENT LITERATURE 4 describes an electrode having an undoping capacitance (B) of 337 to 449 mAh/g, and an initial efficiency (B/A) of 35.1% to 66.7%, by electrochemically pre-doping lithium to a negative electrode active material where the weight ratio is 25% to 100%. PATENT LITERATURE 4 also describes a lithium ion capacitor having a negative electrode to which lithium is pre-doped electrochemically in an amount of 400 mAh/g to 700 mAh/g, to a negative electrode active material where the weight ratio is 31.6% to 69.7%.
PATENT LITERATURE 5 describes an electrode having an undoping capacitance (B) of 312 to 456 mAh/g, and an initial efficiency (B/A) of 27.1% to 66.7%, by electrochemically pre-doping lithium to a negative electrode active material where the weight ratio is 16.3% to 77.3%. PATENT LITERATURE 5 also describes a lithium ion capacitor having a negative electrode in which lithium is pre-doped electrochemically in an amount of 500 mAh/g to a negative electrode active material where the weight ratio is 46.4%.
PATENT LITERATURE 6 describes a lithium ion capacitor having a negative electrode in which lithium is pre-doped electrochemically in an amount of 700 mAh/g to 1500 mAh/g to a negative electrode active material where the weight ratio is 62% to 97%. It is clear from the lithium ion capacitor described in PATENT LITERATURE 6 that durability evaluated by a float charging test is enhanced by controlling a pre-doping amount of lithium ions.